Swine Flu: Just the Latest Chapter in a 91-Year Pandemic Era

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Swine Flu: Just the Latest Chapter in a 91-Year Pandemic Era

The current strain of H1N1 influenza, or swine flu, has people scared because it's a novel virus that most of the population has never been exposed to. But as a group, H1N1 viruses aren't new. They've been circulating since 1918, when a new strain appeared simultaneously in pigs and humans and killed 40 to 50 million people in a single year.

Over the past 91 years, the virus has jumped back and forth between humans, pigs and birds – and possibly even been resurrected from a laboratory freezer. Taking a historical view of the swine flu is critical to understanding the current pandemic, and future outbreaks, argue scientists in two perspectives published Monday in the New England Journal of Medicine.

"Ever since 1918, this tenacious virus has drawn on a bag of evolutionary tricks to survive in one form or another, in both humans and pigs, and to spawn a host of novel progeny viruses with novel gene constellations," wrote scientists from the National Institute of Allergy and Infectious Diseases in one* perspective.*

They say we've been living in a "pandemic era." From 1918 to 1957, H1N1 viruses circulated every year as the seasonal flu. Then, in 1957, a new flu virus, H2N2, appeared on the scene and H1N1 disappeared.

"When a new strain comes out, it overwhelms the population," said Shanta Zimmer, an infectious disease expert at the University of Pittsburgh and a co-author of the other perspective. "Because people had no existing immunity, the new virus replicated and H1N1 got immunologically squeezed out."

Nineteen years later, H1N1 resurfaced on a military base in Fort Dix, New Jersey, where it infected 230 soldiers and killed one. Although the 1976 outbreak was quickly contained and never made it off the base, fear of a worldwide pandemic prompted a massive research campaign and immunization program.

Zimmer calls the H1N1 epidemic that followed "a self-fulfilling prophecy." In November 1977, the strain showed up in Hong Kong, the Soviet Union, and northeastern China. But when scientists examined the genetic make-up of the 1977 virus, it didn't look like versions from the most recent outbreaks. Instead, it resembled a strain from 1950, which (according to the University of Pittsburgh researchers) probably came from a laboratory freezer.

"From 1957 to 1977 we didn't see any H1N1," Zimmer said, "just H2N2. Then it reemerged in 1977, kind of out of the blue. It's circumstantial evidence, but the sudden reemergence coupled with the virus' genetic profile makes us think it came from the accidental release of a 1950 strain."

Of course, there could be other explanations for the mysterious reappearance. Flu surveillance was not nearly as robust in the 1970s as it is today, said epidemiologist Ian Lipkin of Columbia University, a member of the World Health Organization's surveillance network.

"There could have been strains circulating that we didn't know about," he said. "It's fine to speculate, but based on genetics alone, we don't know for sure that it came from a lab."

Scientists also recently confirmed that flu viruses can survive in frozen lakes, which could be another reason why old viruses sometimes reappear suddenly.

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As bad as a "pandemic era" sounds, Zimmer says that the fact that H1N1 flu strains have been around for so long may explain why the 2009 pandemic hasn't hit as hard as scientists first feared.

"Genetic components of the new pandemic strain have their roots in the original 1918 virus, and that’s part of the hope," she said. "We don't understand yet how much immunity there’s going to be, but the fact that it's been less devastating than we feared may be a factor that's related to pre-existing immunity."